Make Clang the default compiler in Makefile and CMakeLists.txt

We switched to Clang18 as reference compiler in our CI. Both the Makefile and CMakeLists.txt still default to gcc as default compiler. Currently, one needs to pass CLANG=0 to make if one wants to build with clang (without sanitizers). With CMake, it is currently not possible to build with Clang without using any sanitizer.